Cisco Switching/Routing :: 6509 / 4900 - Network Down Cause Switching Supervisor With Redundancy

we've had an issue with our network, we have 2 6509 connected with redundancy, which are connected with 2 x 4900 Switches, from which are connected to a ESX Chassis for visualization, the thing is that the ESX stopped working, and the 4900 switches, and the main core were suffering from overload, they hang on it very well, in order to stop the overload, one of the links to the ESX Chassis were disconnected from one of the 4900 switches. The CPU usage from the 4900 and the core(6509) went down below 40%, and then they started to migrate the virtual servers from the chassis to another 2 chassis that were added right after. They were actually working well, but suddenly the 6509 changed to the other supervisor after everything was OK. We were wondering what could have been the cause of this, maybe the virtual servers migrations, maybe the overload from the ESX ? We also had a few question, is there any need to reload the cores every few months as a planned task ? Because the cores have been up for more than 1 year. And also is there any kind of of tool to monitor the CPU status, or the status overall from the cores or the switches ?